With its current business model (no ads, not selling anything), HN's revenue is zero. But it has non-zero costs: hosting fees, salaries of two moderators, etc. So its value as a business would be negative.

If you were to start running HN as a profit-making company, its nature would change so significantly that it wouldn't really be "HN" anymore, and many of the participants would go elsewhere. And the participants are what give HN its value.